Final answer:
The property or theorem that can be used to prove that vertical angles are congruent is the Vertical Angles Theorem.
Step-by-step explanation:
In geometry, the property or theorem that can be used to prove that vertical angles are congruent is the Vertical Angles Theorem. According to this theorem, when two lines intersect, the angles formed opposite each other, known as vertical angles, are congruent.
For example, if we have two intersecting lines and angles A and B are vertical angles, then A = B. This can be proven using the properties of angles and lines.